1. Field of the Invention
The present invention relates to a technique which allows conducting a doping process or other chemical and physical treatments efficiently even at a low temperature.
2. Prior Art
Known processes for doping semiconductors with impurities include a diffusion process and an ion implantation process. The diffusion process comprises heating the semiconductor to a high temperature in the range of from 1000 to 1200xc2x0 C. to make the impurities diffuse into semiconductors. In an ion implantation process, a predetermined portion of a semiconductor is bombarded with an ionized impurity which has been accelerated in an electric field.
The diffusion coefficient D of an impurity can be expressed with an exponential function of absolute temperature T as D=D0xc2x7exp[xe2x88x92Ea/kT], where D0 is the diffusion coefficient at T=∞, Ea is the activation energy, and k is the Boltzmann constant. This equation describes the increase of diffusion coefficient with elevating temperature; accordingly, it has been common practice to carry out diffusion at temperatures as high as possible, preferably, at 1000xc2x0 C. or higher. In the ion implantation process, on the other hand, it is necessary to activate the impurity and to remove the defects in the crystal lattice damaged by the ion bombardment; i.e., the implantation is followed by high-temperature annealing in the temperature range of from 600 to 950xc2x0 C.
Recently, some types of active-matrix liquid crystal display devices using a thin-film transistor (TFT) provided on a glass substrate as the switching device have brought into practical use. The source and drain regions in the TFTs of those display devices are, in general, formed monolithically with the ohmic contacts using amorphous silicon having either of the N-type and P-type conductivity. Because the TFT used in this case is of an inverse stagger type, it likely produces a parasitic capacitance ascribed to its structure. To prevent this unwanted capacitance from developing, there has been made studies on making use of a TFT having its source and drain being formed in a self-aligned structure. However, the source and drain can be formed in a self-aligned manner only by the use of an ion implantation or ion shower process. Then again, a post annealing at the temperature range of from 600 to 950xc2x0 C. should be carried out to activate the impurities and to recover the damage. Taking into consideration that the general purpose economical glass resists only up to a temperature of about 600 to 700xc2x0 C., those ion implantation and ion shower processes are not feasible in an industrial operation.
As another means to circumvent the problem concerning the recover of thermal damage on the glass substrates, there is known a technology, i.e., impurity doping using a laser beam irradiation. There is known, for example, a process which comprises first covering the intended portion of the surface of the semiconductor with a thin film of the impurity, and then irradiating a laser beam thereto to melt the thin film of the impurity simultaneously with the surface of the semiconductor. In this manner, it is possible to dissolve the impurity into the surface of the molten semiconductor.
In the process above using an excimer laser beam irradiation, the impurity doping can be carried out without causing thermal damage on the glass substrate. However, the process requires an additional step of coating the semiconductor with the impurity. Conventionally, a coating process such as spin coating has been used for this step. However, the quality of this coating is process-determining, because the concentration of the doped impurity depends on the evenness of this coating. Thus, this process is far from being an ideal one. Furthermore, this coating is formed generally using an organic solvent as the solution medium. The use of such an organic solvent sometimes allows unfavorable elements such as carbon, oxygen, and nitrogen to enter into the semiconductor to impair the properties thereof.
In the light of the circumstances described above, the present invention has been achieved with an aim to provide a laser-beam doping technology using particularly an excimer laser, said technology being composed of simplified process steps and free from invasion of foreign elements into the semiconductor during the process. Accordingly, the present invention provides, with an object to simplify the process and to prevent inclusion of undesirable elements, a doping process using a high purity doping material in its gas phase in the place of the conventional solid or liquid phase doping materials. It is another object of the present invention to increase the doping efficiency.
Still other objects of the present invention include doping of elements into, in addition to semiconductors, various types of materials inclusive of insulators and conductors, as well as modifying materials and surfaces thereof. There can be specifically mentioned, for example, doping of phosphorus into a silicon oxide film.
The present invention provides an impurity doping process for imparting either of the N-type and P-type conductivity to the sample semiconductor, which comprises irradiating a laser beam to the surface of a semiconductor sample in a high purity reactive gas atmosphere containing an impurity which renders the semiconductor N-conductive or P-conductive. It is known, however, based on the acquired knowledge of the present inventors, that the process at temperatures as low as the room temperature is yet to be improved to achieve sufficient diffusion of the elements. In the process of the present invention, the laser beam is irradiated to the semiconductor with the semiconductor being maintained at a temperature higher than room temperature.
An embodiment according to the present invention provides, accordingly, a process which comprises heating the sample and maintaining it to at least 200xc2x0 C. during the irradiation of a laser beam, thereby accelerating diffusion of the impurity elements and to dope the semiconductor with the impurity at a high concentration. The temperature to which the substrate is to be heated depend on the type of the semiconductor, and is in the range of from 250 to 500xc2x0 C., preferably from 300 to 400xc2x0 C., in the case of polysilicon (polycrystalline silicon) and semi-amorphous silicon.
Thus heating the semiconductor is not only advantageous for the diffusion of the impurities, but also the semiconductor itself more readily recovers the temporarily lost high crystallinity due to laser beam irradiation, because heating the sample provides thermally a sufficient relaxation time. A sample without being heated and subjected to an irradiation of a laser beam, particularly to a beam of a laser operating in a pulsed mode, experiences a typical rapid heating and rapid cooling. Hence, such samples are apt to turn into an amorphous state. More specifically, the sample is instantaneously heated to a temperature as high as 1000xc2x0 C. or even higher, but is then cooled to room temperature during the next period of several hundreds of nanoseconds. If we consider a case in which the sample is silicon and in which the sample is heated to the temperature range above, the time necessary to reach the lower limit of the crystallization temperature, i.e., about 500xc2x0 C., is calculated to be 10 times as long as that necessary to cool the sample to room temperature. If the duration of laser beam irradiation exceeds a certain duration at this step, the silicon melts to develop a convection which carries the impurities deep into the internal of the silicon. On the other hand, if a pulsed laser beam does not endure for a certain time, the silicon crystallizes into a solid to give a so-called semi-amorphous phase. In this case, the impurities undergoes solid-phase diffusion to enter the internal of the silicon.
It is unfavorable to heat the semiconductor to an excessively high temperature. At too high a temperature, the reactive gas itself undergoes pyrolysis (decomposition by heat) to form deposits not only on the sample but also on the holder and the like. As a result, the efficiency of gas usage may be greatly impaired.
It is also undesirable to maintain the semiconductor at a temperature higher than the crystallization temperature thereof. This is particularly so in the case of semiconductors comprising defects at high density, such as polycrystalline semiconductors, amorphous semiconductors, and semi-amorphous semiconductors. If the doping were to be taken place on a crystalline semiconductor being heated to a temperature of crystallization temperature or higher, the control for valence electrons is almost lost due to the generation of energy levels. Accordingly, it is preferred that the process is conducted by heating the substrate at a temperature not higher than the temperature at which amorphous silicon undergoes thermal transition to polysilicon, i.e., from 500 to 550xc2x0 C., and more preferably, at a temperature not higher than a temperature lower than the transition temperature by 100xc2x0 C. (i.e., about 400 to 450xc2x0 C., or lower). In the case of a TFT using amorphous silicon (referred to hereinafter as a-Si:TFT), the device is destroyed if the temperature exceeds 350xc2x0 C. Thus, such a-Si:TFTs should be maintained at a temperature lower than 350xc2x0 C. Such a care should be taken to other semiconductors as well.
Another embodiment according to the process of the present invention provides a technology for doping of an impurity from a gas phase using a laser, particularly an excimer laser, in which a plurality of elements are doped using different types of doping gases. An object of the present process using a single laser beam is to avoid the drop in doping efficiency due to the use of various doping gases differing in light absorption properties and in decomposition behavior. Accordingly, in the present process comprising irradiating a laser beam to the sample in a reactive gas atmosphere containing an impurity which imparts either of the N- and P-conductive types to the semiconductor, an electromagnetic energy is applied to said reactive gas simultaneously with the laser irradiation to thereby decompose the reactive gas. The doping efficiency can be further improved by heating the semiconductor to a pertinent temperature in the same manner as in the first embodiment of the present invention. For example, this heating is carried out at a temperature not higher than the crystallization temperature of the semiconductor under the application of the electromagnetic energy.